In copending application Ser. No. 917,832, filed concurrently herewith, there is described a nickel-chromium-iron alloy which has strong mechanical properties and, at the same time, has swelling resistance under the influence of irradiation and low neutron absorbence. As such, it is particularly adapted for use as a duct and cladding alloy for fast breeder reactors.
By reference to the aforesaid copending application Ser. No. 917,832, it will be seen that the high strength of the alloy at high temperatures is due to a morphology of the gamma-double prime phase enveloping the gamma-prime phase and in which any delta phase is distributed at or near the grain boundaries. While aging, the alloy described therein will precipitate as three different phases, namely a high temperature delta phase which tends to nucleate and grow at or near the grain boundaries, the gamma-prime spheroidal strengthening phase, and the gamma-double prime platelet strengthening phase. It is desirable, in order to obtain best mechanical properties, to precipitate only the gamma-prime and gamma-double prime phases with the delta phase, in or near the grain boundaries.